208 Messrs. Wright and Thompson on the Determination of 
value is algebraically the greatest,-as was found with cadmium- 
silver and mercury. 
It is hence evident that whilst the thermovoltaic constants 
of metals in contact with their oxy salts (sulphates, nitrates, 
and acetates) do not vary widely, the same remark is not true 
for their halogen salts (chlorides, bromides, iodides). In 
general the thermovoltaic constant is decreased (algebraically) 
by substitution of bromide for iodide or of chloride for 
bromide ; although this rule is not without exception, e. g. 
in the case of iron, where the values for :25 FeX, 100 H,O 
are +°204 and +:°134, where X represents chlorine and 
bromine respectively, the substitution of chlorine for bromine 
thus causing a notable increase. 
Summary of Results. 
207. The experiments above described lead to the general 
conclusion that electromotors, consisting of voltaic cells in 
which two different metals are surrounded by solutions of 
corresponding salts respectively, are capable of generating 
electromotive forces which (when not depreciated sensibly 
below their maximum value by giving rise to currents of 
densities greater than certain small limiting values) usually 
stand in no simple relationship to the chenucal action taking 
place in the cell during the passage of the currrent, or to the 
heat-evolution taking place during that passage, or to the 
heats of formation of the two solutions electrolyzed. The 
values of the maximum H.M.F’.’s thus generated may, however, 
be deduced within very close limits of accuracy by taking the 
algebraic difference between certain numerical values or 
voltaic constants assigned to each metal in contact with each 
given solution, these values varying slightly according to the 
surface-nature of the immersed metal, and being also depen- 
dent on the nature and strength of the solution of metallic 
salt in contact with the metal, and probably also varying with 
the temperature, but being otherwise actually constant. 
Further, these maximum H.M.F. values may be connected 
with the difference in heat of formation between the two 
solutions electrolyzed by supposing that the total difference of 
potential set wp is due to two superposed causes:—one, the heat- 
evolution due to the difference in heat of formation of the two 
solutions, which tends to make the metal immersed in the 
electrolyte of lesser formation-heat acquire the higher potential 
(like the copper plate of a Daniell cell) ; the other, a thermo- 
voltaic action akin to the medus operandi of an ordinary 
